A conventional glow plug used for preheating a diesel engine or the like is comprised of a metal sheath tube having a closed front end. A sheath heater is accommodated in the sheath tube together with a coil, used as a heating element, and insulating powder.
A front end portion of the coil disposed in the sheath tube is joined to the front end of the sheath tube, and a rear end portion of the coil is joined to a front end of a conductive terminal axis that is inserted in the sheath tube rear portion. The coil is energizable through the conductive terminal axis.
The above-mentioned sheath heater is generally manufactured as follows. First, a front end of a cylindrical tube is made in a tapered shape. The coil is then connected to the front end of the conductive terminal axis is disposed in the tube. Thereafter, one end of the coil is welded to the front end portion of the tube, and the front end portion of the tube is closed. Then, insulating powder, such as magnesia, is filled in the tube, and a sealing is provided between the rear end of the tube and the conductive terminal axis so as to seal the tube. Thereafter, the sheath tube is subjected to a swaging step. The thus-produced sheath heater is assembled into a metal shell with a projecting manner to complete the glow plug.
However, since the coil is relatively soft, it may bend or become eccentric during the swaging step. In some cases, a winding pitch of the coil becomes inconsistent. When the coil is greatly bent, the sheath tube and the coil are likely to be in contact each other, causing a short-circuit at the time of energization. As a result, the coil cannot reach at a predetermined temperature. Further, there is also the possibility that the glow plug has a large variation in temperature-rising characteristic of the heater due to inconsistency of the winding pitch of the coil.
Thus, the conventional technology shows that a rod-like insulator is inserted in the coil prior to the swaging step so as to increase density in the sheath tube and prevent the above-mentioned failure (e.g., Japanese Patent Application Laid-Open (kokai) No. 2004-340562).
However, as in a sheath heater 50 shown in FIG. 9, when a rod like insulator 51 having a small diameter is inserted in a coil 52, the coil tends to be greatly bent because a clearance between the insulator 51 and an inner circumference of the coil 52 is relatively large. Therefore, an insulator 61 with a relatively large diameter is preferably employed so that the clearance with the inner circumference of the coil 52 may be reduced, as shown in FIG. 10.
However, as shown in FIG. 10, the diameter of the front end portion of the coil 52 is tapered so as not to contact with a front end side tapered portion 53a of a sheath tube 53. Therefore, if an insulator 61 having a large diameter is employed, and the coil 52 is bent in a small degree, the insulator 61 cannot reach a front end of a taper-shaped reduced diameter portion 52a of the coil 52. In this case, only insulating powder fills the vicinity of the front end portion of the coil 52, and density in the vicinity of the front end portion becomes relatively low. As a result, when the front end portion of the coil 52 is subjected to the swaging step, the coil 52 is likely to deform locally or to have an inconsistent thickness in the vicinity where the insulator 61 is not inserted in coil 52. This is because the sheath tube deforms in the swaging step, and an impact (i.e., force) is exerted on the coil as a result of movement of the insulating powder due to the deformation during the swaging step. If the coil is greatly deformed at a particular location, the thickness of the coil may become inconsistent at that location. Particularly, resistance in a thin portion of the coil becomes large. As a result, the coil tends to heat up at locations of deformation, causing a disconnection (failure) at an early stage.
An object of the present invention is to overcome the above-mentioned problems and to provide a glow plug capable of preventing a deformation or eccentricity of the coil, improving durability and reducing a variation in temperature-rising characteristic.
Another object of the present invention is to provide a method for manufacturing the glow plug.